1. Field of the Invention
The present invention relates to a backlight system and particularly to a multi-lamp backlight system with only a single transformer, with driving currents of the lamps inherently balanced.
2. Description of the Prior Art
Present displays, such as LCD, require a backlight system of small size and high power efficiency. Thus, CCFL (Cold Cathode Fluorescent Lamp) is popularly used in the backlight system. Moreover, backlight systems with only a single lamp are not suitable for large size displays requiring much higher illumination. Only multi-lamp backlight systems have illumination performance to satisfy this requirement.
FIG. 1 is a diagram showing a conventional multi-lamp backlight system. It includes a driving circuit 11, a transformer composed of a primary coil set 121, a secondary coil set 122 and a core 123, capacitors C1 and C2, current balancing circuits 13, and lamps 151 and 152. The driving circuit 11 provides a low AC voltage applied to the primary coil set 121 of the transformer 12 so that a high AC voltage is induced on the secondary coil set 122, by which the lamps 151 and 152 are supplied with power. Since the resistances or parasitic capacitances of lines connecting the transformer 12 to the lamps 151 and 152 can be different, the magnitudes of thus generated driving currents of the lamps 151 and 152 can also differ. This results in illumination difference between the lamps 151 and 152, which degrades the performance of the display. The current balancing circuits 13 eliminate the current difference of the lamps 151 and 152.
FIG. 2 is a diagram showing another conventional multi-lamp backlight system. For sake of clarity, the same elements in FIGS. 1 and 2 refer to the same symbols. The systems shown in FIGS. 1 and 2 are similar, except that the current balancing circuits 23 in FIG. 2 are coupled between the lamps 151 and 152, and ground.
FIG. 3 is a diagram showing the structure of the transformer 12 used in conventional multi-lamp backlight systems. The primary and secondary coil set 121 and 122 are both wrapped around and respectively disposed on two sides of the core 123.
With the conventional transformer structure, the lamps in the backlight system must be connected in series or parallel when using one single transformer, or connected to one transformer each when using multiple transformers since there is only one secondary coil set. However, the transformer suffers much high voltage on its secondary coil set with the lamps serially connected, additional current balancing circuits are needed when the lamps are connected in parallel, and a backlight system with multiple transformers has a high cost and occupies a large space in the display.